The recovery of C1 to C4 carboxylic acids (hereinafter “lower acids”) from aqueous streams is a common industrial problem arising from a variety of reaction and processing steps. Simple distillation of wet acid streams to recover glacial acids is hampered by unfavorable vapor-liquid equilibrium (VLE) and high energy costs with all C1 to C4 carboxylic acids. Examples of unfavorable VLE include the formic acid-water maximum-boiling homogeneous azeotrope, the acetic acid-water VLE “pinch” (a region of low relative volatility), and the minimum-boiling homogeneous azeotropes with water and all C3-C4 carboxylic acids.
Various approaches have been suggested in the art to address the problem of lower acid recovery from wet acid feeds. For example, one approach subjects an aqueous lower acid solution to azeotropic distillation together with an entraining component capable of forming a heterogeneous minimum-boiling azeotrope with water, so that the azeotrope boils at a temperature substantially lower than pure water, the pure lower acid, and any acid-water azeotrope. An extraction step often precedes the azeotropic distillation. The extraction step partitions the carboxylic acid into a water-immiscible solvent (which is often the same as the azeotropic entrainer) in order to remove the bulk of the water from the recovered acid. Many examples of azeotropic distillation, extraction, and combinations thereof using conventional organic solvents have been proposed in the art. These include U.S. Pat. Nos. 1,839,894; 1,860,512; 1,861,841; 1,917,391; 2,028,800; 2,050,234; 2,063,940; 2,076,184; 2,123,348; 2,157,143; 2,184,563; 2,199,983; 2,204,616; 2,269,163; 2,275,834; 2,275,862; 2,275,867; 2,317,758; 2,333,756; 2,359,154; 2,384,374; 2,395,010; 2,537,658; 2,567,244; 2,854,385; 3,052,610; and 5,662,780, and Eaglesfield et al., “Recovery of Acetic Acid from Dilute Aqueous Solutions by Liquid-Liquid Extraction—Part 1,” The Industrial Chemist, Vol. 29, pp. 147-151 (1953).
Several solvent characteristics determine the capital and energy costs of extraction-distillation processes for the extractive recovery of lower acids from wet acid feeds. The solvent for the extraction process is immiscible with water and meets two criteria:                a) The solvent shows some selectivity between extraction of the carboxylic acid and water, i.e., the ratio of carboxylic acid to water in the extraction solvent after extraction is substantially larger than in the wet acid feed stream. This factor can be quantified as the weight ratio of water to acid in the extract stream as defined in more detail below.        b) The solvent shows sufficient affinity and capacity for the lower carboxylic acid.These characteristics are quantifiable from experimentally determined equilibrium partition coefficients as defined in more detail below.        
The equilibrium partition coefficient (also used interchangeably with the term “partition coefficient”) for component A (the lower carboxylic acid) is defined as follows:
      P    ⁡          (      A      )        =            weight      ⁢                          ⁢      percent      ⁢                                        ⁢                                      ⁢      A      ⁢                          ⁢      in      ⁢                          ⁢      solvent      ⁢                          ⁢      phase              weight      ⁢                          ⁢      percent      ⁢                          ⁢      A      ⁢                          ⁢      in      ⁢                          ⁢      aqueous      ⁢                          ⁢      phase      
The partition coefficient is a measure of the relative concentrations of the solute to be extracted in the two phases. The value of the acid partition coefficient is directly related to the amount of solvent that is required to effect a given extraction. Low values of the partition coefficient indicate high levels of solvent are required, and high values of the partition coefficient indicate low levels of solvent are required. Since the acid partition coefficient changes with acid concentration, the minimum amount of solvent required to effect a given amount of acid extraction also changes. Thus, the controlling solvent flow requirement for the extraction is dictated by the lowest value of the acid partition coefficient as the acid concentration varies from the high of the inlet wet acid feed to the low of the outlet acid concentration of the exiting raffinate stream.
The controlling acid partition coefficient may be defined as:Pcont=minimum(Praff,Pextr)where
Praff=acid partition coefficient at an acid concentration approaching that desired in the raffinate stream (i.e., at low acid concentration); and
Pextr=acid partition coefficient at an acid concentration approaching that desired in the extract stream (i.e., at high acid concentration).
The most important water-acid selectivity value is that at the extract end of the extraction cascade. It is defined as:Rextr=Wextr/Aextr where
Wextr=weight fraction of water in the extract product stream; and
Aextr=weight fraction of acid in the extract product stream.
The controlling partition coefficient, Pcont, and extract water-to-acid ratio, Rextr, may be combined to yield an overall extraction factor, ε, which is a simple measure of the efficacy of a given solvent for recovering lower acids from wet acid feeds in an extraction-distillation process. The extraction factor, ε, is defined as:ε=Pcont/Rextr=(Pcont*Aextr)/Wextr 
Generally, the higher the extraction factor, the lower the capital and energy costs are for a given extraction.
Extraction solvents that exhibit the inverse behavior are also known. That is, their acid partition coefficient is lowest at the extract end of the cascade (high acid concentration) and highest at the raffinate end (low acid concentration). Examples of such solvents include nitriles, phosphate esters, phosphine oxides (U.S. Pat. Nos. 3,816,524 and 4,909,939), and amines (e.g., King, “Amine-Based Systems for Carboxylic Acid Recovery: Tertiary Amines and the Proper Choice of Diluent Allow Extraction and Recovery from Water,” CHEMTECH, Vol. 5, pp. 285-291 (1992); and Tamada et al., “Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretation of Data,” Ind. Eng. Chem. Res., Vol. 29, pp. 1327-1333 (1990)).
This inverse behavior (partition coefficient highest at low acid concentration) has also been observed for a phosphonium- and an ammonium-phosphinate ionic liquid (Blauser et al., “Extraction of butyric acid with a solvent containing ammonium ionic liquid,” Sep. Purif. Technol., Vol. 119, pp. 102-111 (2013); Martak et al., “Phosphonium ionic liquids as new, reactive extractants of lactic acid,” Chem. Papers, Vol. 60, pp. 395-98 (2006)) and a phosphonium carboxylate salt (Oliveira et al., “Extraction of L-Lactic, L-Malic, and Succinic Acids Using Phosphonium-Based Ionic Liquids,” Sep. Purif. Tech., Vol. 85, pp. 137-146 (2012)). Oliveira et al., in particular, report that P666,14-decanoate fails to form a simple two-phase system in the majority of extraction trials with lactic, succinic, and malic acids. A third phase complicates the distribution and recovery of carboxylic acid.
The use of hydrophobic ionic liquids as extraction solvents has been reviewed by Poole et al., “Extraction of Organic Compounds with Room Temperature Ionic Liquids,” J. Chromatogr. (A), Vol. 1217, pp. 2268-2286 (2010). The development and advantages of phosphonium ionic liquids have been reviewed by Robertson et al., “Industrial Preparation of Phosphonium Ionic Liquids”, Green Chem., Vol. 5, pp. 143-152 (2003)), and their application to the extraction of ethanol from fermentation broths is addressed by Neves et al., “Separation of Ethanol-Water Mixtures by Liquid-Liquid Extraction Using Phosphonium-Based Ionic Liquids,” Green Chem., Vol. 13, pp. 1517-1526 (2011).
Extraction of lower carboxylic acids using imidazolium and phosphonium ionic liquids has also been reported. For acetic acid, McFarlane et al. report on bmim-NTf2, omim-NTf2, bmim-PF6, P666,14-LABS/nonanol, P444,14-LABS/nonanol, and P666,14-OSO2Me (“Room Temperature Ionic Liquids for Separating Organics from Produced Waters,” Sep. Sci. & Tech., Vol 40, 1245-1265 (2005)). Hashikawa claims P222,8-NTf2 for acetic, propionic, and butyric acids (“Method for Producing Acetic Acid,” JP 2014/40389, Daicel, (Mar. 6, 2014)). And Matsumoto et al. report on bmim-PF6, hmim-PF6, and omim-PF6 (“Extraction of Organic Acids Using Imidazolium-Based Ionic Liquids and Their Toxicity to Lactobacillus rhamnosus,” Sep. and Purif. Tech., Vol. 40, pp. 97-101 (2004)).
None of these documents, however, employed a quaternary phosphonium carboxylate with mono-functional carboxylic acids. Moreover, lower carboxylic acid partitioning was poor in the reports by McFarlane, Hashikawa, and Matsumoto. Furthermore, adding an alcohol to phosphonium ionic liquid compositions for extracting lower carboxylic acids is recognized by those skilled in the art as not being preferred, due to the formation of carboxylic ester derivatives of the alcohols with the acid extracts, especially in downstream distillation or evaporative processes for purifying the lower carboxylic acids. This exclusion is addressed by Judson King, “Acetic Acid Extraction,” Handbook of Solvent Extraction, Krieger Publ. Co. (1991).
Hashikawa, in particular, claims only using ionic liquids with fluorine-containing anions, such as bis(fluorosulfonyl)imide, bis(fluoroalkylsulfonyl)imides, tris(perfluoroalkyl)trifluorophosphates, hexafluorophosphates, tetrafluoroborates, and perfluoroalkylsulfonates. These anions add significant cost and toxicity concerns to large-scale applications. In addition, Hashikawa claims ionic liquids with phosphonium salts containing a total of only ten carbon atoms or higher. According to the data presented in the Hashikawa application, triethyl(octyl)phosphonium bis(trifluoromethylsulfonyl)-imide exhibits relatively poor extraction behavior for acetic acid, with a small two-phase region, low capacity for acetic acid, and very low partition coefficients (between about 0.06 and 0.1).
Despite the poor performance of the above reported and claimed ionic liquid systems, the extremely low vapor pressure of ionic liquids remains an attractive physical property for a lower-carboxylic-acid-extracting phase. Thus, there is a need in the art for an extraction solvent with excellent partitioning of lower carboxylic acids from aqueous solutions and that enables the simple separation of these acids via distillation from the solvent. There is also a need for extraction solvents with high extraction factors whereby C1 to C4 carboxylic acids can be recovered from wet acid feeds in an energy-efficient and cost-effective manner.
The present invention addresses these needs as well as others, which will become apparent from the following description and the appended claims.